1. Field of the Invention
The present invention generally relates to an apparatus and method for allocating resources in a wireless communication system and a system using the same, and in particular, to an apparatus and method for allocating resources in a wireless communication system using Frequency Division Multiple Access (FDMA), and a system using the same.
2. Description of the Related Art
Wireless communication systems have been developed to allow users to perform location independent communication. A wireless communication system providing a voice service is a typical wireless communication system. With the rapid progress of communication technologies, wireless communication systems providing voice service are developing to provide data service.
In wireless communication systems, research on various methods has been conducted for data service, and studies have been conducted on a method for providing data service using FDMA. In particular, extensive research has been conducted on Orthogonal Frequency Division Multiplexing (OFDM), a kind of FDMA, to provide a high-speed data service not only in a wireless system but also in a wired system. OFDM, a scheme for providing data using multiple carriers, is a kind of Multi-Carrier Modulation (MCM) that converts a serial input symbol stream into parallel symbol streams, and modulates them with orthogonal sub-carriers, i.e. sub-carrier channels, before transmission. An OFDM-based system that distinguishes several users with the sub-carriers, i.e. an OFDM-based system that supports several users in the manner of allocating different sub-carriers to different users, is generally called Orthogonal Frequency Division Multiple Access (OFDMA).
With reference to FIG. 1, a description will now be made of an example in which resources are allocated in an OFDMA system. FIG. 1 shows an example of transmitting data with allocated resources by a Mobile Station (MS) in a general OFDMA system.
In FIG. 1, reference numeral 101 in a lattice denotes a particular resource which is composed of one or multiple sub-carriers in the frequency domain, and is composed of one or multiple OFDM symbols in the time domain. The parts hatched by slanted lines indicate resources allocated for data transmission by a first MS (MS1), and the parts hatched by double-slanted lines indicate resources allocated for data transmission by a second MS (MS2). As used herein, the term ‘resources’ refers to resources in the time and frequency domains, and indicates OFDMA symbols in the time axis and sub-carriers in the frequency axis. The resources used by the MS1 and the MS2 for data transmission continuously use specific frequency bands without time variation. This resource allocation scheme or data transmission scheme selects a frequency region having a good channel state and allocates resources in the selected frequency domain to each MS, thereby maximizing system performance with limited system resources.
For example, as to the wireless channel that the MS1 experiences, the parts indicated by slanted lines in the frequency domain are better than other frequency domains. However, as to the wireless channel that the MS2 experiences, the parts indicated by double-slanted lines in the frequency domain are better than other frequency domains. A scheme of selecting the frequency region having a good channel response from the frequency domains and allocating resources in the selected frequency domain is generally called a ‘frequency selective resource allocation’ or ‘frequency selective scheduling’ scheme. Although the foregoing description has been made for an uplink, i.e. data transmission from an MS to a Base Station (BS) for convenience, by way of example, the same can also be applied to a downlink, i.e. data transmission from a BS to an MS. For the downlink, the parts hatched by slanted lines and the parts hatched by double-slanted lines indicate the resources used by the BS to transmit data to the MS1 and the resources used by the BS to transmit data to the MS2, respectively. However, the frequency selective scheduling is not always available. For example, for an MS moving at high speed, the frequency selective scheduling is unavailable due to a fast change in the channel state. The reason is as follows. After a BS scheduler selects a frequency region having a better channel state for a particular MS and allocates resources in the selected frequency domain to the MS, the channel environment may have already changed considerably at the time the MS receives resource allocation information from the BS and actually transmits data using the allocated resources. Therefore, it cannot guarantee that the selected frequency region has a better channel state. In this case, a frequency hopping scheme is available. It should be noted that the use of the frequency hopping scheme is not limited to the nonuse of the frequency selective scheduling scheme.
FIG. 2 shows an example in which a frequency hopping scheme is used in a general OFDMA system. Reference numeral 201 denotes a particular resource which is equal to that denoted by reference numeral 101 in FIG. 1.
Referring to FIG. 2, it is noted that resources used by one MS for data transmission suffer continuous variation (hopping) in the time domain. This frequency hopping process contributes to randomization of channel quality and interference that the data transmission experiences.
However, the use of only the frequency hopping scheme in OFDMA or the scheme of allocating a particular frequency band for a predetermined time cannot increase the resource efficiency.
Hybrid Automatic Repeat reQuest (HARQ) technology is one of the major technologies used for increasing data transmission reliability and data throughput in the general wireless communication system. HARQ refers to a combined technology of Automatic Repeat Request (ARQ) and Forward Error Correction (FEC). ARQ, technology popularly used in wire/wireless data communication systems, refers to technology in which a transmitter assigns a sequence number to a transmission data packet according to a predetermined scheme and transmits the packet, and a data receiver sends to the transmitter a retransmission request for a packet with a missing number among the received packets using the numbers, thereby achieving reliable data transmission. FEC refers to the technology that adds redundant bits to transmission data according to a rule and transmits the data, like convolutional coding or turbo coding, thereby overcoming noises occurring in a data transmission/reception process and errors occurring in the fading environment. In this manner, FEC demodulates the originally transmitted data. In a system using HARQ, which is the combined technology of ARQ and FEC, a data receiver determines presence/absence of errors by performing a Cyclic Redundancy Check (CRC) check on the data decoded through an inverse FEC process on the received data. If the CRC check result indicates absence of error, the data receiver feeds back an Acknowledgement (ACK) signal to a transmitter so the transmitter may transmit the next data packet. However, if the CRC check result indicates presence of error in the received data, the data receiver feeds back a Non-Acknowledgement (NACK) signal to the transmitter so the transmitter may retransmit the previously transmitted packet. The receiver combines the retransmitted packet with the previously transmitted packet, thereby obtaining energy and coding gain. HARQ, compared with the conventional ARQ not supporting the combing, can obtain higher performance.
FIG. 3 shows an example in which data is transmitted based on HARQ. In FIG. 3, the horizontal axis indicates a time axis. Blocks 301, 302, 303 and 311 each show transmission of one sub-packet. That is, a general HARQ system transmits several sub-packets in order to successfully transmit one packet. A number shown in each block indicates an identifier for a corresponding sub-packet. For example, a sub-packet indicated by ‘0’ is a sub-packet, which is initially transmitted in a process of transmitting one packet. If the sub-packet indicated by ‘0’ is first transmitted, a data receiver receives the sub-packet and then attempts demodulation thereon. It is shown in FIG. 3 that demodulation of the first transmitted sub-packet is failed. That is, if it is determined that there is an error in the data transmission, the receiver feeds back a NACK signal. A transmitter receiving the NACK signal transmits the next sub-packet, i.e. a sub-packet indicated by a sub-packet identifier ‘1’. Upon receipt of the sub-packet with a number ‘1’, the data receiver combines the sub-packet #0 with the sub-packet #1, and then reattempts the demodulation. It is shown in FIG. 3 that the data receiver fails in the demodulation even on the combined sub-packet of the sub-packet #0 and the sub-packet #1. Therefore, the receiver feeds back a NACK signal again because there is an error in the data transmission. The above process is repeated until the transmission sub-packet is successfully received at the data receiver, or repeated until the transmission reaches a maximum number of retransmissions. It is shown in FIG. 3 that the receiver succeeds in decoding when it receives a sub-packet #2 corresponding to third transmission. In order to prevent the receiver from continuously transmitting the same sub-packet due to the continuous failure in the decoding of the sub-packet, a particular system may limit the number of retransmissions.
HARQ is popularly used in wireless communication systems, as a very useful method. Therefore, there is a need for the use of HARQ for resource allocation even in wireless communication systems using OFDMA (OFDMA wireless communication systems).